Nonaqueous phase organic liquids are widespread contaminants of subsurface soils and groundwater. From a human health perspective, these organic liquids are of particular concern because they tend to represent long-term sources of pollution and create diverse and persistent pathways for eventual human exposure. The work proposed in this Project is directed towards the investigation of processes which influence the entrapment, persistence, transport, bioavailability, and toxicity of organic liquid components in the natural subsurface environment. It represents a fundamental research program which builds upon previous and current NIEHS-funded projects and integrates laboratory and mathematical modeling investigations. This project focuses on the behavior and toxicity of complex polycyclic aromatic hydrocarbon (PAH) mixtures. Its specific aims relate to the quantification of compositional influences on phase equilibria, mass transfer kinetics, and biotransformations, and to the exploration of sorption processes and bioavailability in natural media. Collaboration with other project investigators through the Products Evaluation Core will facilitate the assessment of the epigenetic toxicity of PAH mixture prior and subsequent to biotransformation. Thus, the work proposed is designed to logically extend the knowledge gained in previous NIEHS-funded projects to consider the interaction of multiple processes, the influence of chemical and physical heterogeneities on these processes, and the behavior and fate of complex NAPL mixtures. Although directed primarily at the investigation of naturally occurring attenuation processes, the knowledge gained from these studies will have immediate applications to the design of more effective remediation strategies.